Method for controlling modem

ABSTRACT

When the present modem control apparatus detects that the receive level of a probing signal in phase 2 of the pre-communication procedure or a tone signal with a specific frequency in phase 2 of the pre-communication procedure is lower than a prescribed value, it sets the transmission rate of the control channel to a low rate of 1200 bps. In this case, the modem control apparatus sets the transmission rate to 1200 bps through a prescribed bit of the MPh signal in the communication control procedure and performs the setting that will not permit communications with the answer modem at different transmission rates, through the second prescribed bit of said MPh signal, and notifies those settings to the answer modem, thus carrying out communications at a lower transmission rate.

TECHNICAL FIELD

The present invention relates to modem control methods, especially thoseapplicable to a V.34 facsimile modem communicating in compliance withthe T.30 ANNEX-F procedure based on the ITU (InternationalTelecommunication Union) V.34 facsimile modem Recommendation.

BACKGROUND ART

According to the communication protocol based on the current V.34Recommendation and T.30 Recommendation ANNEX-F, modems operate infull-duplex mode in a control channel where communication procedures areexchanged and operate in half-duplex mode in a primary channel totransmit image information. FIG.5 shows the communication protocol.

Phase 1 is a section where a CM signal (calling menu signal) and JMsignal (common menu signal) are exchanged. This section selects amodulation mode available to the calling and answer modems.

Phase 2 is a section called “line probing.” L1 and L2 are signals toprobe the line characteristic viewed from the answer modem eachincluding 21 single-frequencies from 150 Hz to 3750 Hz. INFO is aninformation signal indicating communication capacity and signals A, Abar, B, and B bar indicate the ACK (Acknowledge) signals that receiveINFO and are signals to adjust the transmit/receive timing of signal L1and signal L2.

Phase 3 is a primary channel preparation stage which transmits V.34image signal data which will be issued in a later stage and correspondsto the period of transmission of a long training signal (longsynchronization signal). The frequency band (or symbol rate) used inthis long training signal is determined based on the line characteristicprobed by the L1 and L2 signals in phase 2.

The control channel includes two groups of signals; first-half section Aand second-half section B. Section A is mainly for exchanging operatingparameters of the modem and it is in this section that the transmissionrate of the image signal data transmitted through the primary channelwhich follows the control channel is determined. Section B is a sectionto exchange control information as the facsimile terminal and it is inthis section that control commands such as DIS (Digital IdentificationSignal) and DCS (Digital Command Signal) described in the T.30Recommendation are exchanged.

An MPh signal in section A contains a bit (bit 50 in MPh) whichdetermines whether or not to accept asymmetric transmission rates of1200 bps and 2400 bps as the transmission rate of section B. The MPhsignal also includes another bit (bit 27 in MPh) which requests theanswer modem to transmit section B at either 1200 bps or 2400 bps.

If bit 50 in MPh of both the transmitter and receiver is “1”,communication is carried out at an asynchronous transmission rate and ifbit 50 in MPh of either the transmitter or receiver is “0”,communication is carried out at the same transmission rate.

For both 1200 bps and 2400 bps, the control channel at the calling modemuses a 1200 Hz carrier and a band of 1800 Hz or lower, while the controlchannel at the answer modem uses a 2400 Hz carrier and a band of 1800 Hzor higher.

However, the T. 30 ANNEX-F currently does not accept asymmetriccommunications (communications carried out at different transmissionrates between the calling and answer modems) in FAX transmissions andstipulates that MPh bit 50 is set to “0”.

When a symmetric rate transmission is selected, if the requestedtransmission rate declared by bit 27 in the MPh signal differs betweenthe calling and answer modems, it is stipulated that the transmitter andreceiver should carry out communications in section B according to theslower transmission side.

In a current FAX which incorporates a V.34 modem, the transmission rateof section B is preset to either 1200 bps or 2400 bps and it is possibleto select either one according to the line situation.

The primary channel contains facsimile image information data and ashort training signal (short synchronization signal) which precedes thefacsimile image information data. The data signal rate of this imageinformation part is determined by the exchange of the aforementioned MPhsignal of section A of the control channel.

FIG. 6 is a section diagram of a conventional receive circuit thatreceives facsimile control signals. A receive signal passes through AGC(Automatic Gain Control) circuit 1, A/D converter 2 and is input to QAMdemodulator 3, 1200 Hz detection circuit 4, 2400 Hz detection circuit 5and FFT circuit 6.

QAM demodulator 3 is a QAM (Quadrature Amplitude Modulation) demodulatorwhich detects the modem performance of phase 2, line probing result andINFOoc and INFOoa, etc. of an INFO sequence used to exchange data mode(image information transmission mode) modulation parameters, anddemodulates them.

1200 Hz detection circuit 4 and 2400 Hz detection circuit 5 detect 1200Hz and 2400 Hz signals to capture signals in section B and section A inphase 2.

FFT circuit 6 is a fast Fourier transformation circuit which analyzesthe frequency characteristic of line probing signals L1 and L2 in phase2 and determines the symbol rate (frequency band) of a primary channelsignal used in phase 3 and thereafter.

However, the conventional technology above has the following problems.

The transmitter transmits image information after selecting the optimumdata signal rate according to the line situation by evaluating the linecharacteristic using the line probing signal. The transmitter fixes thedata transmission rate in control channel section B at either 1200 bpsor 2400 bps set beforehand.

In the case of 1200 bps, noise resistance is considerably high to suchan extent that it can be an excessive quality for normal lines, whereasits data transmission rate is low. On the other hand, with 2400 bps, theopposite is the case. Therefore, transmission of the control channelwith a noisy line often ends up repeating data retransmission. Thedifference of noise resistance between 1200 bps and 2400 bpscommunications of the control channel is approximately 7 dB.

In communications of the control channel above, the symbol transmissionrate is as low as 600 baud, and thus it has a strong resistance in linelink characteristics.

However, since band separation type full-duplex communications are usedfor the control channel, the receive signal level in either a high bandof 1800 Hz or higher or a low band of less than 1800 Hz remains as lowas close to −43 dBm specified in the T.4 Recommendation. On the otherhand, a signal transmitted by the calling modem may be reflected fromthe exchange and introduced to the demodulator as an echo. Since thegain of this echo signal may be greater than the receive signal level,either the calling modem or answer modem will have lower reception noiseresistance. Therefore, in the case of a transmission rate of 2400 bps,there is a problem of extremely high probability of bit errors inreceive data of the receiver.

DISCLOSURE OF INVENTION

The present invention has been implemented in view of the problemsabove. Its objective is to provide modem control methods which willallow the transmission rate of the control channel to be selectedaccording to the line situation.

The other objective is to make it possible to select the transmissionrate of the control channel reliably and simply using the proceduredefined in the ITU Recommendation.

These objectives are achieved by a modem operating according to theRecommendation V.34 procedure by setting the transmission rate of thecontrol channel to as low as 1200 bps when it detects that the receivelevel of the probing signal in phase 2 of the pre-communicationprocedure or a tone signal at a specific frequency in phase 2 is lowerthan a prescribed value.

These objects are also achieved by the modem setting the transmissionrate to 1200 bps using the prescribed bit of the MPh signal and usinganother prescribed bit of the MPh signal to perform the settings thatwill not permit communications with the answer modem at differenttransmission rates, notifying the answer modem of those settings, thusimplementing communications at a lower transmission rate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a section diagram showing the overall configuration of themodem control apparatus that implements the modem control method of thepresent invention;

FIGS. 2A and 2B shows explanatory diagrams of probing signals L1 and L2;

FIG. 3 is a section diagram of the receive circuit in phase 2 of thefacsimile control procedure implementing the modem control method inEmbodiment 1 of the present invention;

FIG. 4 is a section diagram of the receive circuit in phase 2 of thefacsimile control procedure implementing the modem control method inEmbodiment 2 of the present invention;

FIG. 5 is a schematic diagram showing a V.34 facsimile modem and overallT.30 ANNEX-F procedure; and

FIG. 6 is a section diagram of the receive circuit in phase 2 of thefacsimile control procedure implementing a conventional modem control.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the attached drawings, the embodiments of thepresent invention are explained below.

FIG. 1 is a section diagram that gives an outline of the entire modemcontrol apparatus that implements the modem control method of thepresent invention.

Modem control apparatus 100 comprises receive section 101, communicationprocedure control section 102, receive signal energy detection section103, reference level storage section 104, signal level comparisonsection 105, transmission rate control signal generator 106, transmitsection 107, and image information processing section 108.

The pre-communication procedure is switched by communication procedurecontrol section 102, receive section 101 and transmit section 107. Forreceive signal energy detection section 103, a reference signal with aspecific frequency in the pre-communication procedure input from receivesection 101 is input. This receive signal energy detection section 103analyzes the receive level of the input reference signal and outputs itto signal level comparison section 105. This signal level comparisonsection 105 compares the receive level of the analyzed reference signalwith a prescribed value stored in reference level storage section 104and judges its receive level. The judgment result is input totransmission rate control signal generator 106, which in turn carriesout the following processing according to the judgment result.

If the receive level of the reference signal is higher than theprescribed value, transmission rate control signal generator 106continues to exchange control signals of the control channel at the sametransmission rate. On the other hand, if the receive level of thereference signal is lower than the prescribed value, transmission ratecontrol signal generator 106 generates a control signal which will setthe transmission rate to a lower value and sends it to the answer modemthrough communication procedure control section 102.

Thus, the pre-communication control procedure thereafter will be carriedout at a lower transmission rate. When the pre-communication controlprocedure is completed, image information processing section 108 carriesout transmission/reception with the answer modem.

The reference signal with a specific frequency above can be, forexample, line probing signals L1 and L2 in phase 2 of thepre-communication control procedure of the V.34 facsimile modemoperating in compliance with the ITU Recommendation T.30 ANNEX-F.

FIG. 2 shows the L1 and L2 signals in phase 2 and (A) indicates thesignal level of the calling modem and (B) indicates the signal level ofthe answer modem. As shown in the figures, the L1 and L2 signals eachconsist of 21 simultaneously transmitted single frequencies from 150 Hzto 3750 Hz.

The calling modem transmits signals of the same level as shown in (A).These signals attenuate at the answer modem due to the line situation assown in (B). Thus, the answer modem measures the incoming level in alower and higher frequency bands and if the receive signal level isestimated to be lower than the prescribed value in either frequencyband, it requests the calling modem to carry out communications at atransmission rate of 1200 bps.

The modem control methods related to the embodiments of the presentinvention are explained more specifically below.

(Embodiment 1)

FIG. 3 is a section diagram showing the configuration of the receivesection of probing signals L1 and L2 in phase 2 of the V.34 facsimilemodem control procedure that implements the modem control method relatedto Embodiment 1 of the present invention.

A receive signal. passes through AGC (Automatic Gain Control) circuit301, A/D converter 302 and is input to QAM demodulator 303, 1200 Hzdetection circuit 304, 2400 Hz detection circuit 305 and FFT circuit306.

QAM demodulator 303 is a QAM (Quadrature Amplitude Modulation)demodulator which detects the modem performance in phase 2, line probingresult and INFOoc and INFOoa, etc. of an INFO sequence used to exchangedata mode (image information transmit mode) modulation parameters, anddemodulates them.

1200 Hz detection circuit 304 and 2400 Hz detection circuit 305 detect1200 Hz and 2400 Hz signals to capture signals in section B and sectionA in phase 2, respectively.

FFT circuit 306 is a fast Fourier transformation circuit which analyzesthe frequency characteristic of line probing signals L1 and L2 in phase2 and determines the symbol rate (frequency band) of a primary channelsignal used in phase 3 and thereafter.

The circuits above from AGC circuit 301 to FFT circuit 306 are the sameas the circuits described in FIG. 6.

1800 Hz cut-off high pass filter 307 is the filter that allows signalsin a band of 1800 Hz or higher used in the control channel by the answermodem and calling modem to pass. 1800 Hz cut-off low pass filter 308 isthe filter that allows signals in a band of less than 1800 Hz used bythe answer modem and calling modem to pass.

High energy evaluation section 309 measures the energy of signals thatpass 1800 Hz cut-off high pass filter 307. Low energy evaluation section310 measures the energy of signals that pass 1800 Hz cut-off low passfilter 308.

The operation of the receive circuit configured as above is explainedusing the communication protocol diagram in FIG. 5.

First, the data modulation mode used for transmission/reception in phase1 in which a CM signal and JM signal are exchanged is selected. Then, inphase 2, line probing to probe the line characteristic at the answermodem is carried out.

In phase 2, the answer modem receives an INFOoc signal and demodulatesthe INFOoc signal using QAM demodulator 303. Then, the answer modemreceives a B signal and B bar signal using 1200 Hz detection circuit304. These are sent in response to an A signal sent from the answermodem and A bar signal sent from the calling modem. These B signal an Bbar signal are 1200 Hz signals with phase 0 degree and phase 180 degreeand become timing signals to generate a non-signal section to receiveprobing signals such as L1 signal and L2 signal. This allows the answermodem to receive the L1 signal and L2 signal without contention wish thecontrol signal transmitted.

The answer modem then receives the L1 signal and L2 signal and inputsthem to FFT circuit 306 and analyzes the frequency characteristic of theline. From this analysis result the answer modem determines the symbolrate of the V.34 primary channel signal in phase 3.

On the other hand, the L1 signal and L2 signal pass through 1800 Hzcut-off high pass filter 307 and 1800 Hz cut-off low pass filter 308simultaneously and are input to high energy evaluation section 309 andlow energy evaluation section 310.

High energy evaluation section 309 and low energy valuation section 310measure the receive signal energy levels in the high frequency band andlow frequency band of the L1 signal and L2 signal input to determinewhether each level is as low as close to −43 dBm which is the setoperation level of the T.4 Recommendation.

If the measurement result of high energy evaluation section 309 and lowenergy evaluation section 310 shows that at least one of the high energyor low energy side is close to −43 dBm, the answer modem notifies thecalling modem to carry out communications at 1200 bps. To be concrete,this notification is performed by setting bit 27 of the MPh signal insection A of the control channel first-half section after phase 3 to“0”. The calling modem operates so that it receives the control channeldata transmitted at a transmission rate of 1200 bps by analyzing bit 27of the received MPh signal.

Measurements by high energy evaluation section 309 and low energyevaluation section 310 above are only carried out by the answer modem.However, as a general characteristic of a public telephone network, ifthe receive level at the answer modem is low, the receive level of thecalling modem is also estimated to be low. Therefore, if the measurementresult at the answer modem shows that the receive level of either thehigh energy side or low energy side is low, receive noise resistance inthe control channel is likely to be reduced at either the calling modemor answer modem, and thus the transmission rate is switched.

Furthermore, since communications by the above control procedure arecarried out by a facsimile control procedure based on the T.30 ANNEX-F,no asymmetric rate communication (transmission rate differs between thecalling and answer modems) is accepted. The answer modem indicates thecalling modem that no asymmetric rate communications will be carried outby setting bit 50 of the MPh signal in the control channel to “0”. Whenit receives this MPh signal, the calling modem operates so thatcommunication of the control channel may be carried out at atransmission rate of 1200 bps which is highly resistant to noiseirrespective of whether bit 27 of the MPh signal is “0” or “1”.

In the case that the communication cable from the facsimile terminal toa remote station exceeds 10 km, the 2400 Hz carrier used for the controlchannel of the answer modem may attenuate considerably with respect tothe 1200 Hz carrier used for the control channel of the calling modem.The present invention demonstrates its effects particularly in such acase.

(Embodiment 2)

FIG. 4 is a section diagram showing the configuration of the receivesection of probing signals L1 and L2 in phase 2 according to the V.34facsimile modem control procedure which implements the modem controlmethod related to Embodiment 2 of the present invention. FIG. 4 shows areceive circuit of the answer modem and the circuits from AGC circuit301 to FET circuit 306 are the same as the receive circuits described inEmbodiment 1. In Embodiment 2, 1200 Hz detection circuit 304 and 2400 Hzdetection circuit 305 are each followed by an energy evaluation section.This eliminates the necessity of providing a 1800 Hz cut-of f high passfilter 307 and 1800 Hz cut-qff low pass filter separately as in the caseof Embodiment 1.

Energy evaluation section 411 is a circuit for evaluating energy of thereceive signal detected by 1200 Hz detection circuit section 304 andenergy evaluation section 412 is a circuit for evaluating energy of thereceive signal detected by 2400 Hz detection circuit section 305.

The receive circuits above operate as follows:

In FIG. 4, QAM demodulator 303 of the answer modem receives an INFOocsignal in phase 2 and demodulates that signal. Then, 1200 Hz detectioncircuit 304 receives a tone B signal and tone B bar signal. The tone Band tone B bar signals are normally 1200 Hz timing signals with phase 0degree and phase 180 degree, respectively as explained above.

After detecting the tone B bar signal, the answer modem receives the L1signal and L2 signal and analyzes the frequency characteristic of theline through FFT circuit 306. Then, it determines the symbol rate of theprimary channel signal shown in FIG. 4 according to the frequencycharacteristic of the analyzed line. At the same time, it inputs thereceived data of tone B and tone B bar signals to energy evaluationcircuit 411 and measures the energy level of low frequency band receivesignals.

On the other hand, the calling modem also has a receive apparatussimilar to the receive apparatus shown in FIG. 4, performing the samecontrol as the apparatus of the answer modem. That is, QAM demodulator303 of the answer modem receives an INFOoc signal in phase 2 anddemodulates it. Then, the answer modem receives a tone A signal and toneA bar signal through 2400 Hz detection circuit 305. After the tone A barsignal is detected, the answer modem transmits the tone B signal andtone B bar signal, and waits the reception of the tone A signal againwhile transmitting the L1 and L2 signals. In parallel with theseoperations, it inputs the receive data of the tone A signal and tone Abar signal to energy evaluation circuit 412 and measures the energylevel of high frequency band receive signals.

In this way, the calling modem measures the 2400 Hz signal, while theanswer modem measures the 1200 Hz signal, examining whether each signalis as low as close to −43 dBm which is the minimum operation levelspecified by the T.4 Recommendation. When either calling modem or answermodem detects that the receive signal level is close to −43 dBm, eithermodem which has detected that the receive level is low in section A ofthe first-half of the control channel will set bit 27 of the MPh signalto “0” and request the opposite modem for communications through thecontrol channel at 1200 bps.

Since the entire control procedure is applicable to communications basedon the T.30 ANNEX-F, the modem that has detected that the level of thereceive signal from the opposite modem sends an MPh signal by settingbit 50 of the Mph signal which declares asymmetric rate communication to“0”. The opposite modem that has received this MPh signal cannot helpbut respond to communications at a transmission rate of 1200 bps, andthus the calling and answer modems will carry out communications throughthe control channel at 1200 bps which is mutually highly resistant tonoise.

As seen above, the present invention allows communications at 1200 bpswhich is highly noise resistant on a noisy line, while communicationsusing the control channel at high rate of 2400 bps when there is littlesignal attenuation.

INDUSTRIAL APPLICABILITY

The modem control methods in the present invention are suitable fortransmitting/receiving image information using V.34 facsimile modems andespecially suitable for facsimile apparatuses using telephone lineswhose line quality is likely to deteriorate.

What is claimed is:
 1. A modem control method applicable to a V.34facsimile modem which operates in compliance with the ITU RecommendationT.30 ANNEX-F for a calling modem and an answering modem mutuallyconnected via a communication line, the modem control method comprising:receiving a signal having a specific frequency inserted in acommunication control procedure; comparing a level of the receivedspecific frequency signal with a predetermined value; carrying outcommunications at a lower transmission rate when the comparing indicatesthat the level of the received signal is lower than the predeterminedvalue, wherein the specific frequency signal includes probing signals L1and L2 in a pre-communication procedure transmitted from the callingmodem, and wherein both first setting of a transmission rate and secondsetting that determines whether communications between the calling modemand the answering modem are permitted be at different transmission ratesare carried out by assigning the first and second settings to differentbits in a MPh signal of a control channel, the MPh signal being used forexchanging operating parameters between the calling and answering modemsand for determining a transmission rate of signals transmitted through aprimary channel that follows the control channel, and wherein, when acurrent transmission rate of the control channel is 2400 bps, the MPhsignal requests the calling modem to communicate at a transmission rateof 1200 bps.
 2. A modem control method applicable to a V.34 facsimilemodem which operates in compliance with the ITU Recommendation T.30ANNEX-F for a calling modem and an answering modem mutually connectedvia a communication line, the modem control method comprising: receivinga signal having a specific frequency inserted in a communication controlprocedure; comparing a level of the received specific frequency signalwith a predetermined value; carrying out communications at a lowertransmission rate when the comparing indicates that the level of thereceived signal is lower than the predetermined value, wherein, whenlevels of probing signals L1 and L2 received from the calling modem arelower than the predetermined value, carrying out of communications atthe lower transmission rate comprises: setting the lower transmissionrate to 1200 bps through a first predetermined bit of a MPh signal in acontrol channel, the MPh signal being used for exchanging operatingparameters between the calling and answering modems and for determininga transmission rate of signals transmitted through a primary channelthat follows the control channel; prohibiting communications between thecalling modem and the answering modem at different transmission rates,through a second predetermined bit of the MPh signal; and notifying thecalling modem of the setting of the lower transmission rate and of theprohibition of communications at different transmission rates.
 3. Themodem control method according to claim 2, wherein the firstpredetermined bit is bit 27 of the MPh signal and the secondpredetermined bit is bit 50 of the MPh signal.
 4. A modem control methodapplicable to a V.34 facsimile modem which operates in compliance withthe ITU Recommendation T.30 ANNEX-F for a calling modem and an answeringmodem mutually connected via a communication line, the modem controlmethod comprising: receiving a signal having a specific frequencyinserted in a communication control procedure; comparing a level of thereceived specific frequency signal with a predetermined value; carryingout communications at a lower transmission rate when the comparingindicates that the level is lower than the predetermined value, whereinthe specific frequency signal is one of 1200 Hz and 2400 Hz tone signalsin a pre-communication procedure transmitted from the answering modem,wherein both first setting of a transmission rate and second settingthat determines whether communications between the calling modem and theanswering modem at different transmission rates are permitted arecarried out by assigning the first and second settings to different bitsin an MPh signal of a control channel, the MPh signal being used forexchanging operating parameters between the calling and answering modemsand for determining a transmission rate of signals transmitted through aprimary channel that follows the control channel, and wherein, when acurrent transmission rate of the control channel is 2400 bps, the MPhsignal requests the answering modem for communications at a transmissionrate of 1200 bps.
 5. A modem control method applicable to a V.34facsimile modem which operates in compliance with the ITU RecommendationT.30 ANNEX-F for a calling modem and an answering modem mutuallyconnected via a communication line, the modem control method comprising:receiving a signal having a specific frequency inserted in acommunication control procedure; comparing a level of the receivedspecific frequency signal with a predetermined value; carrying outcommunications at a lower transmission rate when the comparing indicatesthat the level of the received signal is lower than the predeterminedvalue, wherein if a level of one of 1200 Hz and 2400 Hz tone signalsreceived from the calling modem is lower than the predetermined value,carrying out of communications at the lower transmission rate comprises:setting the lower transmission rate to 1200 bps through a firstpredetermined bit of an MPh signal in a control channel, the MPh signalbeing used for exchanging operating parameters between the calling andanswering modems and for determining a transmission rate of signalstransmitted through a primary channel that follows the control channel;prohibiting communications between the calling modem and the answeringmodem at different transmission rates, through a second predeterminedbit of the MPh signal; and notifying the calling modem of the setting ofthe lower transmission rate and of the prohibition of communications atdifferent transmission rates.
 6. The modem control method according toclaim 5, wherein the first predetermined bit is bit 27 of the MPh signaland the second predetermined bit is bit 50 of the MPh signal.
 7. A modemcontrol apparatus which is provided for communication between a callingmodem and an answering modem mutually connected via a communication lineand is applicable to a V.34 facsimile modem which operates in compliancewith the ITU Recommendation T.30 ANNEX-F, the modem control apparatuscomprising: an energy detector that receives a reference signal having aspecific frequency inserted in a communication control procedure andthat detects energy of the reference signal; a comparator that comparesa level of the detected energy of the reference signal with apredetermined reference level; a controller that transmits a controlsignal to carry out communications at a lower transmission rate when aresult of the comparison by the comparator indicates that the level ofthe reference signal is lower than the predetermined reference level,wherein the specific frequency reference signal includes one of probingsignals L1 and L2 in a pre-communication procedure transmitted from thecalling modem and one of 1200 Hz and 2400 Hz tone signals in thepre-communication procedure transmitted from the answering modem, andwherein the controller sets a transmission rate to 1200 bps through afirst predetermined bit of an MPh signal in a control channel, prohibitscommunication between the calling modem and the answering modem atdifferent transmission rates through a second predetermined bit of theMPh signal, and notifies the answering modem of the lower transmissionrate and of the prohibition of communication at different transmissionrates, so as to carry out communications at a lower transmission rate,the MPh signal being used for exchanging operating parameters betweenthe calling and answering modems and for determining a transmission rateof signals transmitted through a primary channel that follows thecontrol channel.
 8. A facsimile apparatus that includes the modemcontrol apparatus of claim
 7. 9. A modem apparatus for communicationbetween a calling modem and an answering modem, comprising: an energydetector that receives a reference signal from the calling modem in acommunication control procedure, the reference signal having a specificfrequency, and that detects an energy of the specific frequencyreference signal; a comparator that compares a level of the detectedenergy of the specific frequency reference signal with a predeterminedreference level; a controller that, when the comparison by thecomparator indicates that the detected energy level is lower than thepredetermined reference level and a current transmission rate is ahigher transmission level, transmits a MPh signal of a control channelto the calling modem, the MPh signal including a first bit requestingcommunication at a lower transmission rate, and a second bit indicatingthat communications between the calling modem and the answering modem atdifferent transmission rates are prohibited, the MPh signal being usedfor exchanging operating parameters between the calling and answeringmodems and determining a transmission rate of signals transmittedthrough a primary channel that follows the control channel.
 10. Afacsimile apparatus including the modem apparatus of claim
 9. 11. Themodem apparatus according to claim 9, wherein the higher transmissionlevel is 2400 MHz and the lower transmission level is 1200 MHz.